Measures for securing stability of a ship when it is damaged are required.
For example, a ship such as a passenger boat having a plurality of compartments and provided with a water trap in the hull has been proposed (see, for example, Patent Literature 1), the water trap being structured as a wall element for partitioning adjacent compartments, which is given for the purpose of limiting the amount of flooding water to secure the ship's stability when the ship is damaged; the wall element includes a fire-resistant main wall element that supports a predetermined load, and a fire-resistant additional element that forms a wall partitioning adjacent compartments together with the main wall element and is located under the main wall element, with no liquid filled inside. The additional element blocks movement of a heated gas between the adjacent compartments, while it allows movement of seawater to the other compartment when the seawater enters one of the adjacent compartments.
With the structure described in this Patent Literature 1, when seawater enters one of adjacent compartments due to a hull breach, the water trap causes the seawater to enter the other compartment, too. This brings about a condition in which the plurality of compartments inside the hull are flooded, which suppresses rotational moment around the axis of the hull, whereby the stability performance of the hull is improved.
However, if the above-described water trap is to be applied to a car carrier or the like, there are the following problems: a) There need to be provided a plurality of bulkheads (watertight bulkheads) inside the vehicle stowage compartment, which poses limitations on the compartment design; b) The compartment need to be partitioned by providing watertight sliding doors or the like between vehicle stowage decks; c) Partitioning the vehicle stowage compartment increases the material amount and leads to a cost increase; d) Partitioning the vehicle stowage compartment increases the material amount and leads to an increase in the hull weight (L/W or lift to weight ratio); e) Partitioning the vehicle stowage compartment deteriorates work efficiency during the ship's construction; f) Partitioning the vehicle stowage compartment deteriorates work efficiency of the crew; and g) Limitations will arise on the stowage of vehicles, and the number of vehicles that can be loaded will be decreased.
On the other hand, if no measures are taken, a required value of GoM (transverse metacentric height) will be larger so as to secure the stability of a car carrier or the like when it is damaged. A larger required value of GoM will pose limitations on the stowage of vehicles in the ship service, and such limitations on the vehicle stowage will decrease the scale of operation.
Alternatively, in order to secure the ship's stability when it is damaged, a ballast for lowering the center of gravity would be necessary, which, for a car carrier or the like, will cause a problem that the number of vehicles that can be loaded is decreased.
Another type of ship has also been proposed (see, for example, Patent Literature 2), which includes a flat-bottomed main hull and a submerged tank auxiliary hull that is constituted by adding, under the bottom of the main hull, a submerged tank external hull having an opening for transmitting pressure of incoming and outgoing seawater and an air vent pipe. A joining and securing keel (fin keel) is provided between the lower part of the bottom of the main hull and the interior of the submerged tank auxiliary hull, so that the ship is formed (constituted) by the main hull, which serves as the ship based on conventional ship theories, and the submerged tank auxiliary hull that encompasses a new concept and bears the function of preventing a capsize of the ship. Thereby, the apparent center of gravity is moved to a lower part of the ship so that it is not easily capsized.
However, the one described in Patent Literature 2 is substantially identical to a ship provided with a ballast for lowering the center of gravity, and so it has the problem that the number of vehicles that can be loaded is decreased.
Patent Literature 1: Japanese Published Unexamined Patent Application No. 2004-9950
Patent Literature 2: Japanese Published Unexamined Patent Application No. 7-304490